Why ...

One of the features of PNG is its capability to store images consisting
of RGB color data together with a so called alpha-channel. The
alpha-channel specifies for each pixel the amount of transparency,
or when you like it the other way round the amount of opacity.

Compared to this, the transparency capability of GIF is only an on/off
method. A pixel is transparent or not. With PNG you have the availability
of as many levels of transparency as there are levels of color, which
means in most cases 256 levels.

The use of alpha-channels is three-fold. First of all, you can do all
kinds of fading or shadowing. Just like in real life, many materials
ranging from glass to smoke are visible, but also allow you to see
what is behind it. Second use of alpha channels is the possibility
to create images that are not rectangular. The image itself is of course
still a rectangle, but with the alpha-channel you create a mask that
you can give any shape you like.

Third use is what is called anti-aliasing or sub-pixeling. This addresses
the problem of the wrongly colored outlines around fonts, that we see so
often on the web. Instead of using half-tones for pixels on the edge, the
pixel should be full color, with an alpha value representing the level of
anti-aliasing. This way, images could be displayed against backgrounds of
any color or pattern.

playing around with the menu-options allows various displays

What ...

The latest versions of Netscape, IExplorer and Opera are supporting
PNG images. However, all of them don't handle transparency, alpha and
background colors correctly. Therefore I wrote this small Windows-95/NT
program that demonstrates how proper support of alpha-channels should
be done. AlphaMix allows you to
display PNG images, probably with an alpha-channel or a transparency
chunk, against either a background image or against the color specified
using a background chunk.

Similar to web-browsers, by default the background image will be "tiled"
to fill the whole window. Using the menu this option can be changed
to "centered". When no background image is loaded, when present, the
color of the background chunk will be used. Also this option can be disabled.

To get going, after downloading AlphaMix
and extracting the zip-file, start the program and open test_img.png as
foreground image (ctrl-O). You will see the PNG 3-D logo against the
color in the background chunk.
Look how no artifacts of anti-aliasing are visible.
Now open test_bkg.png as the background image (ctrl-B). The background
color disappears and is replaced by a tiled checkboard pattern. Playing
around with the menu options will enable you to get all the different
views shown above.

Finally you are able to save the combination of the (tiled) background
and the (semi-) transparent foreground image into a PNG-file. This output
file itself will not have any alpha-channel or transparency.

two images using a tiledB&W background andan RGBA foreground image

PS

And last but not least, nothing prevents you to use AlphaMix as a straight forward
PNG image viewer for the MS Windows platform. In Windows Explorer you can set
AlphaMix as the default application for PNG files with View | Options | File Types.
After that, double-clicking on any PNG-file will start AlphaMix and show the image.